1. Field of the Invention
The present invention relates to a digital magnetic recording and reproducing ciruit, and, more particularly, to a digital magnetic recording and reproducing circuit which is capable of suppressing asymmetry in such a case as when an erasing head is provided upstream of the recording and reproducing head.
2. Background Technology
In the field of digital magnetic recording and reproducing apparatuses, in recent years, the capacities thereof have become increasingly larger and so improvement of the recording density has been accordingly requested. As a consequence of this trend, a medium to be used has in general a higher coercive force while the overwrite characteristics (hereinafter referred to as OWM) of a conventional recording and reproducing head has been degraded resulting in the complicated generation of a peak shift due to overwrapping with the previous record, thus degrading reliability of the apparatus in question. Due to this fact, such a recording and reproducing head having a large saturated flux density or having a larger leakage flux adjacent to the gap is utilized. However, such a material having a large saturated flux density is limited in use from the viewpoints of durability, economical factor or the like. Furthermore, such a head having a large leakage flux adjacent to the gap, for example a head with a large gap length, exhibits the problematic characteristic of a degraded high frequency. As an example of a head having an improved characteristic is described in the proceedings of the technical presentation, 4aA-10 entitled "Recording and Reproducing Characteristics of a Composite Head" (the 11th Assembly of Japan Applied Magnetics Association), where such a head employs the concept of equivalently varying the length of the gap at the time of recording and reproducing. However, this head is also worse than a conventional head in terms of economy. On the other hand, an example of countermeasures against the peak shift due to a degraded OWM with an erasing head being provided upstream of a recording and reproducing head is disclosed in Japanese Patent Public Disclosure No. 39910/86, Patent Public Disclosure No. 243909/86 and Japanese Utility Model Public Disclosure No. 111016/86. According to the examples disclosed in these Laid-open applications, it can be seen that the OWM is improved in such a head which employs a material having a conventional level of saturated magnetic flux density by performing DC erase by use of an erasing head prior to recording data, and that a random peak shift caused by a previous record may be eliminated. In this case, however, a peak shift having an overlapped DC component (referred to in general as asymmetry) is generated. In other words, even when data in a single cycle are recorded, a difference in peak-to-peak interval of reproduced data is generated depending on whether the record magnetization of data is in the same direction as DC erase or not. Normally, when the magnetization generated by an erasing head is in the same direction as the magnetization generated by a recording and reproducing head, the peak-to-peak interval becomes long, while such an interval becomes short when the above-mentioned magnetizations are in opposite direction. As a result, the reliability of a recording and reproducing apparatus may be reduced. This aspect will be explained by referring to FIGS. 1-3.
FIG. 1 illustrates an example of a digital magnetic recording and reproducing circuit. A magnetic head 1 forms a part of a magnetic circuit and comprises a recording and reproducing core 1a having a gap of a predetermined width and a recording and reproducing coil 1b wound around the recording and reproducing core 1a and having a center tap connected to a power supply V. An erasing head 2 provided at a position upstream of the recording and reproducing head 2 forms a part of a magnetic circuit and comprises an erasing core 2a having a gap width wider than that of the recording and reproducing core 1a and an erasing coil 2b wound around the erasing core 2a and having one end connected to the power source V. To a data input terminal 3, record data in the form of pulses, as shown in FIG. 2b, are input. To a signal input terminal 4, a WRITE gate signal shown at FIG. 2(a) is input. The magnetic recording and reproducing circuit is operable for recording when the WRITE gate signal is at a level low ("L"), while the circuit is inhibited from recording and is operable for reproducing when the WRITE gate signal is at a level high ("H"). An inverter 5 is connected to the WRITE gate signal input terminal 4 and is operable so as to reverse the WRITE gate signal to match the logic. A flip-flop 6 is connected to the data input terminal 3 and adapted to receive record data and produce an output signal (refer to FIG. 2(c)) sequentially reversed in response to the record data pulse to a first output terminal Q. A second output terminal Q of the flip-flop 6 produces a reversed output signal which is in reverse to the output signal appearing at the first output terminal Q. The reversed WRITE gate signal from the inverter 5 is input to a reset terminal C of the flip-flop 6. When the WRITE gate signal is at a level "L", the output signal and the reversed output signal are output from the first and second output terminals Q and Q in accordance with the record data signal. When the WRITE gate signal is at a level "H", the flip-flop 6 is reset so that the outputs from the first and second output terminals Q and Q are kept at a level "L" and a level "H", respectively.
A record driver unit 7 for flowing a recording current through the recording coil 1b comprises a first npn transistor 7a having the collector connected to one end of the recording coil 1b and the base connected to the first output terminal Q of the flip-flop 6, a second npn transistor 7b having the collector connected to the other end of the recording coil 1b and the base connected to the second output terminal Q of the flip-flop 6, and a constant current means 7c connected to the emitters of the first and second transistor 7a and 7b and activated by the reversed WRITE gate signal from the inverter 5. The constant current circuit 7c may comprise, for example, a resistor connected to the emitters of the first and second transistors 7a and 7b, an npn transistor connected between the resistors and the ground and a resistor connected between the base of the npn transistor and the output terminal of the inverter 5. An erasing unit 8 which is activated by the reversed WRITE gate signal from the inverter 5 for causing a constant erasing current to flow through the erasing coil 2b at the time of recording includes an erasing circuit 8a connected between the other end of the erasing coil 2b and the ground, and may comprise, for example, a resistor connected to the other end of the erasing coil 2b and an npn transistor connected between the transistor and the ground and to the base of which an erasing control signal is applied through the resistor. The reversed WRITE gate signal from the inverter 5 may be used as the erasing control signal.
An operation of the magnetic recording and reproducing circuit constituted such as described above will be explained next. Firstly a condition for inhibiting a recording or for reproducing will be explained. For this condition, the WRITE gate signal at a level "H" is reversed by the inverter 5 to a level "L". The "L" level signal is input to the reset terminal C of the flip-flop 6 and disables the constant current circuit 7c. As a consequence, no recording current flows through the recording coil 1b regardless of input recording data. On the other hand, the erasing circuit 8a is disabled by an erasing control signal whereby no erasing current flows through the erasing coil 2b.
In the recording state, the WRITE gate signal is at a level "L". This signal is reversed by the inverter 5 to a level "H" and input to the reset terminal C of the flip-flop 6 and also to the constant current circuit 7c to activate it. As a consequence, the output signals the states of which are sequentially reversed by the pulses of the input recording data are output from the first and second output terminals Q and Q of the flip-flop 6. Namely, the signal from the first output terminal Q changes the state sequentially in response to the reduction of the record data from a level "H" to a level "L", as shown in FIG. 2(c). From the second output terminal Q, a signal in reverse to the signal from the first output terminal Q is output. The first transistor 7a becomes conductive when the signal from the first output terminal Q is at a level "H" and non-conductive when such a signal is at a level "L", the second transistor 7b becomes conductive when the signal from the second output terminal Q is at a level "H" and non-conductive when such a signal is at a level "L". Namely, the second transistor becomes conductive and non-conductive in reverse to the state of the first transistor 7a. As a result, when the first transistor 7a is in the conductive condition, a recording current flows from the power source V via one end of the recording coil 1b, the recording coil 1b, the center tap of the recording coil 1b, the first transistor 7a and the constant current circuit 7c to the ground. When the second transistor 7b is in the conductive condition, a recording current flows from the power source V via the center tap of the recording coil 1b, the recording coil 1b, the other end of the recording coil 1b, the second transistor 7b and the constant current circuit 7c to the ground. Since the recording current flows through the recording coil 1b, a magnetic flux corresponding to the recording current is generated across the gap of the recording coil 1b and makes a record on a recording medium. On the other hand, the erasing circuit 8a is activated by an erasing control signal which flows from the power source V via the erasing coil 2b and the erasing circuit 8a to the ground, whereby a magnetic flux is generated across the gap of the erasing core 2a to erase the previous data recorded on the recording medium, or, magnetize the medium in a constant direction.
A condition of data recorded onto a recording medium at this time will be explained by referring to FIG. 3. The entire width of the previous data recorded on a recording medium 9 (actually, somewhat larger in width than the entire width of data) is erased or magnetized in a constant direction, by the erasing head 2 positioned upstream of the recording and reproducing head 1. Then recording takes place by the recording and reproducing head 1 in accordance with record data, that is, magnetization takes place in the leftward direction indicated by the arrow in FIG. 3(a) when the first transistor 7a is in a conductive state and in the rightward direction as indicated by the arrow in FIG. 3(a) when the second transistor 7b is in a conductive state.
In a magnetic recording and reproducing circuit provided with inexpensive magnetic heads in which an erasing head is located upstream of a recording and reproducing head and constituted such as described above, if record data having equal intervals of pulses are recorded on a record medium, a recording current which flows through the recording coil has a constant cycle as shown in FIG. 2(c), but recorded lengths are different depending on the magnetizing direction as shown in FIG. 3(a) due to a leakage of a magnetic flux from the erasing head 2. As a result, waveforms and pulses reproduced from the recording medium which have been recorded in a manner mentioned above have patterns as shown in FIGS. 3(c) and 3(d). The reproduced pulses assume an equal pulse interval for every other pulse, but adjacent pulse intervals T.sub.1 and T.sub.2 are different, resulting in a larger asymmetry T.sub.3. Besides, a conventional magnetic recording and reproducing apparatus of a large capacity has to employ expensive magnetic heads, resulting in a higher price of the entire apparatus. To the contrary, if inexpensive magnetic heads are used for the apparatus, the reliability of such an apparatus may be reduced.